Additive manufacturing apparatus and method

ABSTRACT

An additive manufacturing apparatus for building objects by layerwise consolidation of material. The apparatus includes a build chamber containing a working area, a plurality of high energy beams for consolidating material deposited in the working area in layers and an optical unit for controlling transmission of the high energy beams onto material in the working area. The optical unit includes a plurality of independently controllable optical elements each optical element for controlling transmission of at least one of the high energy beams onto the material in the working area, the optical unit movable in the build chamber.

SUMMARY OF INVENTION

This invention concerns an additive manufacturing apparatus and method.The invention has particular, but not exclusive, application to scanningmultiple lasers across a powder bed.

BACKGROUND

Additive manufacturing or rapid prototyping methods for producingobjects comprise layer-by-layer solidification of a material, such as ametal powder material, using a high energy beam, such as a laser beam orelectron beam. A powder layer is deposited on a powder bed in a buildchamber and a laser beam is scanned across portions of the powder layerthat correspond to a cross-section of the object being constructed. Thelaser beam melts or sinters the powder to form a solidified layer. Afterselective solidification of a layer, the powder bed is lowered by athickness of the newly solidified layer and a further layer of powder isspread over the surface and solidified, as required.

During the melting or sintering process, debris (e.g. condensate,unsolidified particles of powder etc) is produced within the buildchamber. It is known to introduce a gas flow through the build chamberin an attempt to remove debris from the chamber in the gas flow. Forexample, the M280 model of machine produced by EOS GmbH, Munich, Germanycomprises a series of gas outlet nozzles located in the build chamber tothe rear of the powder bed that pass a flow of gas to a series ofexhaust vents that are located in the build chamber at the front of thepowder bed. In this manner, a planar layer of gas flow is created at thesurface of the powder bed. A similar arrangement is provided inRenishaw's AM250 and AM125 machines, wherein apertures in the buildchamber either side of a powder bed provide substantially planar gasflow across the powder bed.

It is known from DE102005014483 A1 to use four laser beams to scan apowder bed, each laser beam solidifying powder in a different quadrantof the powder bed. Such an arrangement may increase build speed becausedifferent parts of an object or different objects located in differentquadrants can be built simultaneously with different laser beams.

US2013/0112672 discloses an additive manufacturing assembly in which aprimary energy beam is split into a plurality of secondary laser beams.The secondary beams are directed by individually movable energydirecting elements into separate regions of a workspace. A transitassembly may be provided for conveying energy transmitting devices, thetransit assembly comprising a first carriage movable in a firstdirection and a second carriage that moves on the first carriage in asecond direction. Each of the energy transmitting devices emits aseparate laser beam that is independently and separately movable fordirecting energy over separate portions of the part.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided anadditive manufacturing apparatus for building objects by layerwiseconsolidation of material, the apparatus comprising a build chambercontaining a working area, a plurality of high energy beams forconsolidating material deposited in the working area in layers and anoptical unit for controlling transmission of the high energy beams ontomaterial in the working area, the optical unit comprising a plurality ofindependently controllable optical elements, each optical element forcontrolling transmission of at least one of the high energy beams ontothe material in the working area, the optical unit movable in the buildchamber.

Each optical element may be arranged to steer the at least one laserbeam onto material in the working area in a different direction tothat/those in which the optical unit is arranged to move. The differentdirection may be perpendicular to a direction in which the optical unitis arranged to move. The optical unit may be arranged to move in eitherdirection along a linear axis. Each optical element may be arranged toonly steer the at least one laser beam in a direction perpendicular tothe linear axis.

The additive manufacturing apparatus may comprise a control unit forcontrolling movement of the optical unit and optical elements such thatmovement of the laser beam during consolidation of the material isachieved by simultaneous movement of the optical unit and opticalelements.

Each optical element may be arranged to steer the at least one laserbeam in only one-dimension.

Each optical element may be arranged such that movement of the opticalelement can move a spot of the at least one laser beam across theworking surface faster than the spot can be moved across the workingsurface by moving the optical unit.

The plurality of optical elements may be arranged to direct the laserbeams such that, for a position of the optical unit, an entire width ofthe working area can be scanned by steering the laser beams with theoptical elements.

Each optical element may be mounted to rotate about a rotational axis,the rotational axes fixed relative to each other and the optical unit,wherein for a position of the optical unit, an entire width of theworking area can be scanned by steering the laser beams by rotation ofthe optical elements.

The optical unit may comprise at least one laser for generating at leastone of the laser beams, the laser movable with the optical unit.

A scanning zone for each optical element may be defined by a zone overwhich a laser beam can be directed by the independent movement of theoptical element, the optical elements arranged in the optical unit suchthat, for a position of the optical unit, the scanning zones for atleast two of the optical elements overlap.

The apparatus may comprise a control unit for selecting which one of theoptical elements to use to form an area of the object located within aregion in which the scanning zones overlap.

Each optical element may be removably mounted on the optical unit suchthat the optical element can be removed from the optical unit separatelyfrom another one of the optical elements.

Each optical element may be removably mounted on the optical unit usinga kinematic mount.

The movable optical unit may be connected with a gas flow device forgenerating a gas flow across the working area, the optical unit and gasflow device movable as a single unit.

The optical unit may be connected to a wiper for spreading materialacross the working area, the optical unit and wiper movable as a singleunit.

The optical unit may comprise a two-dimensional array of opticalelements.

According to a second aspect of the invention there is provided anoptical unit for an additive manufacturing machine in which objects arebuilt by layerwise consolidation of material, the apparatus comprising abuild chamber containing a working area, the optical unit comprising aplurality of independently controllable optical elements, the opticalunit mountable in a build chamber of the additive manufacturingapparatus to be movable relative to a working area within the buildchamber with each optical element arranged for controlling transmissionof at least one of a plurality of high energy beams onto material in theworking area.

According to a third aspect of the invention there is provided anoptical unit for an additive manufacturing machine in which objects arebuilt by layerwise consolidation of material, the apparatus comprising abuild chamber containing a working area, the optical unit comprising anoptical element mounted within the optical unit so as to rotate aboutonly one axis, the optical unit mountable in a build chamber of theadditive manufacturing apparatus to be movable in a linear directionrelative to a working area within the build chamber with the opticalelement arranged for steering a high energy beam onto material in theworking area.

It will be understood that the term “scan” used herein is not limited tocontinuously running a spot of the high energy beam over a surface butincludes a series of separated discrete exposures (or hops). Forexample, optics may direct the high energy beam to expose a firstlocation to the beam, the beam then turned off and the optics reorientedto direct the energy beam to a second location spaced from the firstlocation when the high energy beam is switched back on. The high energybeam is a beam having sufficient energy to consolidate the material.

Preferably, the apparatus is a selective laser solidification, such asmelting (SLM) or sintering (SLS), apparatus, wherein powder layers aresuccessively deposited across the working area in the build chamber anda laser beam is scanned across portions of each powder layer thatcorrespond to a cross-section of the object being constructed toconsolidate the portions of the powder.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show an additive manufacturing apparatus according to anembodiment of the invention comprising an optical unit for directingmultiple laser beams onto the powder bed;

FIG. 4 is a plan view of an object being formed using multiple laserbeams in accordance with a method of the invention;

FIG. 5 is a plan view of an object being formed using multiple laserbeams in accordance with another method of the invention;

FIG. 6 shows schematically the regions that can be scanned by laserbeams of one embodiment of the apparatus shown in FIGS. 1 to 3;

FIGS. 7a to 7c show a combined optical scanning unit and gas flow deviceaccording to one embodiment of the invention;

FIG. 8 is a plan view of the unit shown in FIGS. 7a to 7 c;

FIG. 9 is a schematic view of a scanning unit comprising arrays ofhorizontally offset optical assemblies for scanning laser beams across aworking area of an additive manufacturing apparatus;

FIG. 10 is a schematic view of scanning unit comprising arrays ofvertically offset optical assemblies for scanning laser beams across aworking area of an additive manufacturing apparatus;

FIG. 11 is a schematic view of an optical unit according to anotherembodiment of the invention;

FIG. 12 shows a mirror according to one embodiment of the invention; and

FIG. 13 shows additive manufacturing apparatus according to a furtherembodiment of the invention.

DESCRIPTION OF EMBODIMENTS

Referring to FIGS. 1 to 3, a laser solidification apparatus according toan embodiment of the invention comprises a build chamber 101 havingtherein partitions 114, 115 that define a build volume 116 and a surfaceonto which powder can be deposited. A build platform 102 defines aworking area in which an object 103 is built by selective laser meltingpowder 104. The platform 102 can be lowered within the build volume 116using a mechanism as successive layers of the object 103 are formed. Thebuild volume 116 available is defined by the extent to which the buildplatform 102 can be lowered.

The apparatus further comprises a movable gas flow device 131. The gasflow device 131 comprises a nozzle 112 and exhaust 110 formed as asingle unit 131 with a fixed distance between a gas inlet 112 a and agas outlet 110 a. A wiper 109 is fixed to the unit 131 and the powderspread across the powder bed 104 simultaneously with movement of theunit 131. The gas nozzle 112 and gas exhaust 110 are for generating agas flow across a part of the powder bed formed on the build platform102. The gas inlet 112 a and gas outlet 110 a produce a laminar flowhaving a flow direction from the inlet to the outlet, as indicated byarrows 118. Gas is re-circulated from the exhaust 110 to the nozzle 112through a gas recirculation loop (not shown) that is also located withinthe chamber 101. A pump (not shown) maintains the desired gas pressureat gas inlet 112 a and gas outlet 110 a. A filter (not shown) isprovided in the recirculation loop to filter from the gas condensatethat has become entrapped in the flow.

Layers of powder 104 are formed as the object 103 is built by powderdispensing apparatus 108 dosing powder to be spread by wiper 109. Forexample, the dispensing apparatus 108 may be apparatus as described inWO2010/007396.

The additive manufacturing apparatus is arranged to scan a plurality oflaser beams over the powder bed 104. In FIGS. 2 and 3, the primary laserbeams from two lasers 105 a, 105 b are fed into splitters 134 a and 134b, each splitter dividing the laser beam into three secondary laserbeams. These secondary laser beams are fed into a movable opticalscanning unit 135 via fibre optical cables 136. The optical scanningunit 135 is for controlling transmission of the secondary laser beamsonto material in the powder bed 104. At any one time all or a subset ofthe secondary laser beams may be used to build the object. (In FIGS. 2and 3, only four secondary laser beams 133 a, 133 b, 133 c, 133 d areshown as active in consolidating powder in the powder bed).

The optical scanning unit 135 is mounted to the build chamber 101 onguides 170 a, 170 b such that optical scanning unit 135 can move ineither direction along one linear axis. The optical scanning unit 135houses a plurality of separate optical assemblies, each one for steeringone of the laser beams onto the powder bed 104.

In this embodiment, each optical assembly comprises focussing optics,such as a pair of movable lenses 138, 139 or an f-theta lens, and asteering optical element, such as a mirror 141 mounted for rotationabout an axis. The mirror 141 is arranged to steer the laser beam alonga line oriented in a first direction perpendicular to the linear axis ofoptical unit 135. In this way, the optical unit 135 can carry out2-dimensional scanning of the powder bed by a combination of linearmovement of the optical unit 135 and rotary movement of the mirrors 141.The optical unit 135 and mirrors 141 are arranged such that through thecombination of movement the entire powder bed 104 can be scanned. Inthis embodiment, the mirrors 141 can direct the laser beams over anentire width (first direction) of the powder bed 104 for any oneposition of the optical unit 135 along the linear axis. Movement of theoptical unit 135 along the linear axis allows scanning to extend acrossthe powder bed 104 in the perpendicular direction. In an alternativeembodiment, an additional movable mirror may be provided in the opticalassembly such that the optical assembly can steer the laser beam over a2-dimensional area.

Because of the small mass of each mirror 141 relative to the larger massof the entire optical unit 135 it is expected that movement of a spot ofthe at least one laser beam across the powder bed in the first directionwill be faster than the speed at which the spot can be moved across thepowder bed through movement of the optical unit.

In an alternative embodiment, the lasers may be integrated into theoptical unit 135.

Computer 160 comprises a processor unit 161, memory 162, display 163,user input device 164, such as a keyboard, touch screen, etc, a dataconnection to modules of the laser sintering unit, such as opticalscanning unit 135 and laser modules 105 a, 105 b, and an external dataconnection 165. The laser modules 105 a, 105 b, optical scanning unit106, flow device 131 and movement of build platform 102 are controlledby the computer 160.

FIG. 4 shows the apparatus being used to scan a powder layer. Each lasercan be directed to any point within a scanning zone 140 a to 140 g. Theflow device 131 moves simultaneously with the optical unit 135 such thatthe laser beams can be directed into the gap between the inlet 112 a ofthe nozzle 112 and the outlet 110 a to the exhaust 110. The splitters134 or optical assemblies each comprise devices 137 for diverting eachsecondary laser beam after splitting into a heat dump such that the beamcan be turned “off” when the laser beam is not required. Accordingly,during scanning, the laser beams are turned on and off and directed tothe required locations between the inlet 112 a and outlet 110 a bymirrors 141.

As can be seen by the blown up section of FIG. 4, the combined movementof the optical unit 135 and mirrors 141 results in a progression/path ofthe spots 155 formed at an angle to the direction in which the opticalunit 135 moves and at an angle to the direction in which the mirrors 141move the laser spots 155. The speeds of the optical unit 135 and themirrors 141 are selected such that, for a scan across the entire widthof a scanning zone 140 a-140 g, the optical unit 135 is moved asufficient distance forward such that when the spot is returned by themirror 141 to a corresponding position in a direction lateral to thedirection of movement of the optical unit 135, the spot 155 b does notoverlap with a previous position 155 a. Each mirror 141 may becontrolled to perform a simple repetitive motion to repeatedly scan thespot 155 across a width of the scanning zone 140 at a set speed, thelaser beam being switched on and off to control which areas of thepowder within the zone 140 are consolidated. In this way, “intelligent”control of the mirrors 141, where the movement of the mirrors 141 iscontrolled such that the spot follows a prescribed path corresponding tothe areas to be consolidated may not be required.

In one embodiment, shown in FIG. 12, the mirror 141, rather than being aflat plate that is rotated back and forth to deflect the laser spotalong the required path, may be a regular polygon, in particular one ofan order higher than four, such as a pentagon, hexagon, heptagon oroctagon, that is rotated in only one direction and positioned relativeto the incoming laser beam 133 such that the laser spot jumps from oneside of the scanning zone to the other when the laser beam traverses acorner of the polygonal mirror 141.

In FIG. 4, adjacent scanning zones are scanned simultaneously. However,as shown in FIG. 5, it may be desirable in a single pass of the powderbed 104 by the optical unit 135 to only scan separated zones 140 a-140 gand to scan the gaps inbetween on one or more further passes of theoptical unit across the powder bed 104.

The scanning zones 140 a to 140 g shown in FIGS. 4 and 5 may overlap inorder that the powder consolidated in each zone can be knitted togetherto form a single object that extends across the zones 140 a to 140 g. Inthese overlapping regions, both laser beams consolidate portions of theobject that fall within these regions. However, in an alternativeembodiment, the scanning zones may overlap more than is necessary toknit the area consolidated in each scanning zone together. Such anarrangement is shown in FIG. 6, wherein scanning zones 140 h to 140 joverlap by a significant amount, such as each scanning zone overlappingat least a quarter of an adjacent scanning zone and preferably half ofthe adjacent scanning zone (in FIG. 4 the zones 140 h to 140 j are shownas having different lengths in the direction of movement of the opticalunit 135 for clarity only and the extent that the optical assemblies canscan the laser beam in this direction is preferably the same for eachassembly). In this way, areas of the powder to be consolidated that fallwithin these overlapping regions can be consolidated by either one ofthe laser beams associated with these scanning zones. Before or during ascanning operation the computer 130 selects which one of the laser beamsto use to scan the area that falls within the overlapping region. Theother laser beam is not used to scan this area, although at interfaceswhere one laser beam “hands-over” to another laser beam, areas of thepowder may be consolidated by both laser beams in order to ensure thatthe separate areas of the object are knitted together.

In an alternative embodiment (not shown), rather than splitting a laserbeam generated by a laser into multiple beams, each laser beam used forconsolidating powder may be generated by a separate laser. Such anembodiment may not comprise splitters 134 or a heat dump. Furthermore,the lasers may be integrated into the optical unit 135.

FIGS. 7a to 7c and 8 show a further embodiment of an optical unit 135and flow device 131. In this embodiment, the optical assemblies 142 a to142 e are mounted on the flow device 131 so as to move therewith. Eachoptical assembly 142 comprises a sealed housing 145 containing a lens139 for focusing the laser beam and an optical element, in thisembodiment a mirror 141, for steering the laser beam onto the powder bed104. The mirror 141 is mounted on a shaft 143 for rotation about an axisunder the control of a motor 144. The housing comprises a connection 146for connecting the housing to an optical fiber that carries the laserbeam. Each optical assembly 142 is separately removably mountable ontothe flow device 131. Accurate positioning of the optical assembly 142onto the flow device 131 is achieved through provision of cooperatingformations 148 a to 148 c and 149 a to 149 c on the housing 145 and theflow device 131, respectively. In this embodiment, the cooperatingformations 148, 149 are a series of kinematic mounts that provide forrepeatable positioning of the optical assembly 142 on the flow device131. In this way, it may be possible to calibrate each optical assemblyoffline and, when an assembly requires replacement, simply remove theassembly from the flow device 131 and plug in a new assembly. In thisway, each assembly is a “plug and play” module requiring limitedinteraction from user to set-up the system.

This embodiment also differs from previous embodiments in that twowipers 109 a and 109 b are provided on the flow device 131. In this way,the flow device 131 can spread powder in both directions. In order toachieve this, powder dispensers may also be provided at either side ofthe powder bed 104.

It will be understood that, in another embodiment, the “plug and play”module design for the optical assemblies may be provided on a carriageseparate from the flow device 131. Furthermore, rather than a movableflow device, the flow device may comprise inlet and outlet nozzles fixedeither side of the powder bed 104.

In a further embodiment, rather than a 1-dimensional array of laserbeams, a two dimensional array of laser beams may be provided. In FIG.9, a two dimensional array of laser beams is provided in a planeparallel to the powder bed 104. For example, the laser beams may beprovided by a series of plug and play modules, as described above. InFIG. 9, a first column 158 of optical assemblies for directing the laserbeams is offset from a second column 159 of optical assemblies. In thisway, the width the scanning zones can be reduced to enable fasterscanning.

In FIG. 10, the laser beams are provided as a vertically stacked2-dimensional array of optical assemblies, wherein the position of onerow 150 of optical assemblies is offset from a position of a second row151 of laser assemblies. Like the embodiment shown in FIG. 9, this mayallow the width of the scanning zones to be reduced. However, the laserbeams all scan along a common line perpendicular to the movement of theoptical unit. This may allow a gap between a gas inlet and gas outlet(not shown) to be small.

In the optical unit 135 of FIG. 10, the laser beams are generated bylaser diodes 153 integrated into the optical unit 135.

It will be understood that the arrays of FIGS. 9 and 10 may be combinedto form a 3-dimensional array of optical assemblies.

FIG. 11 shows another embodiment of the optical unit 135. In thisembodiment, the laser diodes 153 are packed sufficiently close togetherwith the laser beams focussed through microlenses 155 such that theadjacent beams are close enough together to provide melt pools 154(which are typically larger than the 1/e² laser spot diameter) thatcombine to form the object. Accordingly, in this embodiment, there is nosteering optics. The laser diodes are switched on and off to melt thepowder bed as required as the optical unit is moved across the powderbed.

Referring to FIG. 13, apparatus according to a further embodiment of theinvention is shown. This embodiment is similar to the embodiment shownin FIGS. 7a to 7c and FIG. 8 and features of the embodiment shown inFIG. 13 that are the same or similar to those shown in FIGS. 7a to 7cand FIG. 8 have been given the same reference numerals but in the series200.

The embodiment shown in FIG. 13 differs from that shown in FIGS. 7a to7c and FIG. 8 in that rather than each optical assembly/module 142comprising a connection 146 for connecting an optical fibre to theoptical module 142 for delivery of the laser beam, each optical module242 has an opening 246 that is aligned to receive a different laser beam233 delivered by laser modules 205 into the build chamber 201 from aside of the build chamber 201. An appropriate lens 261 may be used tocollimate the laser beam 233 before it is delivered into the buildchamber 201. The laser beams are delivered parallel to the linearmovement of the scanning unit such that the openings 246 remain alignedwith the laser beams 233 as the optical modules 242 are moved across thepowder bed. In this embodiment, the wipers 209 a, 209 b are in the formof rollers.

Modifications and alterations may be made to the embodiments asdescribed herein without departing from the scope of the invention. Forexample, the scanning unit may not extend across an entire width of thepowder bed but may only extend across a partial width of the powder bedbut be movable in two, perpendicular linear directions.

The invention claimed is:
 1. An additive manufacturing apparatus forbuilding an object by layerwise consolidation of material, the apparatuscomprising: a build chamber containing a working area; an optical unitconfigured to control transmission of a plurality of laser beams ontomaterial in the working area for consolidating the material deposited inlayers, the optical unit arranged to move over the working area andcomprising a plurality of optical elements, each optical element movablein the optical unit for scanning a corresponding laser beam of theplurality of laser beams across the material in the working area; and acontrol unit configured to control scanning of the corresponding laserbeam along a path across the working area by controlling (i) movement ofthe optical unit over the working area so that the corresponding laserbeam moves in a first direction, and (ii) movement of the opticalelements in the optical unit, simultaneously with the movement of theoptical unit over the working area, so that the corresponding laser beammoves in a second direction, transverse to the first direction, the pathof the corresponding laser beam being a combination of movement of thecorresponding laser beam in the first direction and the seconddirection.
 2. The additive manufacturing apparatus according to claim 1,wherein the optical unit is arranged to move over the working area alonga linear axis, and each optical element is arranged to steer the atleast one laser beam onto material in the working area in a directiontransverse to the linear axis.
 3. The manufacturing apparatus accordingto claim 2, wherein the transverse direction is perpendicular to thelinear axis.
 4. The additive manufacturing apparatus according to claim1, wherein each optical element is arranged to steer the correspondinglaser beam in only one-dimension.
 5. The additive manufacturingapparatus according to claim 1, wherein each optical element is arrangedsuch that movement of the optical element can move a spot of thecorresponding laser beam across the working surface faster than the spotcan be moved across the working surface by moving the optical unit. 6.The additive manufacturing apparatus according to claim 1, wherein theplurality of optical elements are arranged to direct the laser beamssuch that, for a position of the optical unit relative to the workingarea, an entire width of the working area can be scanned by steering thelaser beams with the optical elements.
 7. The additive manufacturingapparatus according to claim 1, wherein each optical element is mountedto rotate about a rotational axis, the rotational axes fixed relative toeach other and the optical unit, and for a position of the optical unitrelative to the working area, an entire width of the working area can bescanned by steering the laser beams by rotation of the optical elements.8. The additive manufacturing apparatus according to claim 1, whereinthe optical unit comprises at least one laser for generating at leastone of the laser beams, the laser movable with the optical unit.
 9. Theadditive manufacturing apparatus according to claim 1, wherein eachoptical element is removably mounted on the optical unit such that theoptical element can be removed from the optical unit separately fromanother one of the optical elements.
 10. The additive manufacturingapparatus according to claim 9, wherein each optical element isremovably mounted on the optical unit using a kinematic mount.
 11. Theadditive manufacturing apparatus according to claim 1, wherein thecontrol unit is configured to control the movement of the optical unitand the optical elements such that each corresponding laser beam isscanned over the working area along scan paths that are at an angle to adirection in which an optical module moves over the working area.
 12. Anadditive manufacturing apparatus for building an object by layerwiseconsolidation of material, the apparatus comprising: a build chambercontaining a working area; an optical unit configured to controltransmission of a plurality of laser beams onto material in the workingarea for consolidating the material deposited in layers, the opticalunit arranged to move over the working area and comprising a pluralityof independently controllable optical elements, each optical element forsteering a corresponding laser beam of the plurality of laser beams ontothe material in the working area, wherein a scanning zone for eachoptical element is defined by a zone over which the corresponding laserbeam can be directed by the independent movement of the optical element,the optical elements arranged in the optical unit such that, for aposition of the optical unit, the scanning zones for at least two of theoptical elements overlap; and a control unit configured to select asingle one of the at least two optical elements to use to form an areaof the object located within a region in which the scanning zonesoverlap.
 13. An additive manufacturing apparatus for building an objectby layerwise consolidation of material, the apparatus comprising: abuild chamber containing a working area; an optical unit configured tocontrol transmission of a plurality of laser beams onto material in theworking area for consolidating the material deposited in layers, theoptical unit arranged to move over the working area and comprising aplurality of independently controllable optical elements, each opticalelement for controlling transmission of a corresponding laser beam ofthe plurality of laser beams onto the material in the working area; anda gas flow device configured to generate a gas flow across a part of theworking area, and the gas flow device configured to be movable acrossthe working area simultaneously with the optical unit such that thelaser beams are directed to the part of the working area across whichthe gas flow is generated.
 14. The additive manufacturing apparatusaccording to claim 13, comprising: a wiper for spreading material acrossthe working area, the wiper connected to the optical unit such that theoptical unit and the wiper move across the working area as a singleunit.
 15. The additive manufacturing apparatus according to claim 13,wherein the gas flow device comprises at least a gas exhaust for drawingin gas from an atmosphere in the build chamber above the working area.16. The additive manufacturing apparatus according to claim 13, whereinthe gas flow device comprises a gas nozzle and a gas exhaust configuredsuch that gas flow from the gas nozzle to the gas exhaust is across theworking area.
 17. The additive manufacturing apparatus according toclaim 16, wherein each optical element is configured to transmit thecorresponding laser beam to a location on the working area between thegas nozzle and gas exhaust.
 18. The additive manufacturing apparatusaccording to claim 13, wherein the gas flow device is configured togenerate a laminar gas flow parallel to the working area.
 19. Theadditive manufacturing apparatus according to claim 13, wherein theoptical unit is connected with the gas flow device such that the opticalunit and the gas flow device move across the working area as a singleunit.
 20. The additive manufacturing apparatus according to claim 13,wherein the gas flow device is further configured to move with theoptical unit.
 21. The additive manufacturing apparatus according toclaim 13, wherein the optical unit is mounted on the gas flow device.22. An additive manufacturing apparatus for building an object bylayerwise consolidation of material, the apparatus comprising: a buildchamber containing a working area; an optical unit configured to controltransmission of a plurality of laser beams onto material in the workingarea for consolidating the material deposited in layers, the opticalunit arranged to move over the working area and comprising a pluralityof independently controllable optical elements, each optical element forcontrolling transmission of a corresponding laser beam of the pluralityof laser beams onto the material in the working area, wherein theoptical unit comprises a two-dimensional array of the optical elements,and the two-dimensional array of optical elements comprises first andsecond rows of the optical elements, each optical module of the firstrow configured to transmit the corresponding laser beam to acorresponding first region of the working area and each optical moduleof the second row configured to transmit the corresponding laser beam toa corresponding second region of the working area, the second rowlocated above the first row in a direction substantially perpendicularto the working plane and staggered with respect to the first row suchthat the first regions are located adjacent to and interspersed with thesecond regions to provide a continuum of the first and second regionsacross the working area.
 23. The additive manufacturing apparatusaccording to claim 22, wherein the first and second regions overlap inthe working area.